The present invention relates to refractory metals and alloys with enhanced adhesion strength and more particularly to enhanced bonding of the lead wire to the anode.
Electrolytic capacitors used in computers and telecommunications equipment and like high grade applications industry are made of sintered tantalum and niobium powders, including powders made of alloys of these metals with each other and other metals as alternatives to powders made of one or the other of such elemental metals. The capacitors utilizing such anodes have high capacitance per units of volume and weight of the sintered powder porous compact anodes that constitute the anode of the capacitor. The compacts have fine powder sizes of primary particles and uniformity of sizes and porosity of secondary particles (agglomerates) made from the primary particles. The anodes are also characterized in modern usage by miniaturization of the compact as a whole and use of selected additives to the original powders and/or to the compacts to enhance capacitor performance and/or the manufacturing process regarding one or more of capacitance, sinterability and resistance to electrical leakage and voltage breakdown. Known additives for one or more of such purposes include phosphorous, silicon, carbon, nitrogen and other elements.
These anodes have lead wires of niobium, tantalum and of alloys of these metals with each other and other metals. The anode compacts can have pre-placed lead wires that are embedded in the powder and the compacts with such included wires pass through a sinter furnace with the compacts. Alternatively, the compacts can be sintered and lead wires can be welded to the compacts. The compacts can be of rod like or flat polygonal forms and the wires (or round wire of ribbon form) can be adhered to the compact in good structurally supporting and high electrical current/thermal transfer relation by end or side connections.
It is a principal object of the present invention to increase the adhesion strength of refractory metals and alloys to other refractory metals and alloys, such as the bond strength of the lead wire to the anode compact. Good bonding is important for structural reasons just to reliably maintain the integrity and reliability of the anode-lead assembly of a capacitor and of the high value circuit including such capacitor, under the physical, electrical and thermal stress conditions that can occur in the course of capacitor production or use. Even where a gross rupture of the anode to lead wire bond is not detectable a weak bond can lead to higher leakage and/or increased vulnerability to voltage or frequency induced breakdowns.
The problems of the capacitor art related to lead wire bonding are exacerbated at ever smaller sizes of compacts and of the lead wires enabled by high capacitance per unit volume of modern, improved powders and improved compacts.
The present invention provides a method to increase the adhesion strength of refractory metals and alloys to other refractory metals and alloys. In one embodiment of the invention, the bonding of lead wire to capacitor anodes is improved. This is accomplished by modifying the lead wire surface by controlled oxidation and control of wire chemistry and physical properties for optimum use of the oxidation. The oxidation can be done to finish wires or to wire precursors such as bar and rod.
The bond strength between two refractory metal bodies can be measured by several standard methods, such as tensile testing, shear testing or torsional testing. In one embodiment of the invention, a tensile test method is used for measuring the bond strength between the wire and the powder anode. Bond strength is measured by embedding the wire in a powder mass, sintering to form a compact and implementing a consistent tension after sintering to determine force required to pull the wire apart from the compact with and without practice of the present invention and in comparing variants of the present invention. Similarly, electrical characteristics are determined by processing the sintered compact-wire assemblies to finished capacitors (of solid or wet electrolyte types) and conducting standard leakage and breakdown tests under similar conditions of electrical cycling and ambient stress for capacitors whose anode compact-lead wire assemblies were made with and without practice of the present invention and also in comparing variants of the present invention.
The oxidation treatment is applied in a way to produce a depth profile of added oxidation that peaks in a distance of 20 to 200 nanometers from the wire surface. Zero to 200 nanometers is suitable and a peak usually occurs in fact at the surface but as a practical matter the peak is rarely seen at the surface by most measuring systems because of presence at a surface of interfering contaminants. The actual peak or apparent peak (both substantially the same) should have an oxygen level about 20-40% higher (on an atomic % basis) compared to bulk oxygen content level of the wire and in any event above 30 atomic % oxygen at the peak. Roll off from the peak should be steep towards the interior of the wire (50%+attenuation of the differential of peak to bulk oxygen level within 100 nanometers of depth). The manners of oxidation treatment are preferably to pass the wire rapidly through a furnace and expose it to a partial atmosphere of oxygen. But the oxidation can be done in other ways including electrolytic oxidation, wet chemical treatment, pack coating, reactive sputtering, temporary coating (or lamination) and diffusion (and subsequent coating removal). The oxidation is normally done after wire production but can be integrated with wire production. The wire itself can be homogeneous or have a core/sheath configuration by coating or co-extrusion or laminating production.
It has been found that the oxidation substantially enhances wire to compact bond strength while having little or no adverse effect on electrical characteristics of the eventual capacitor. It is surprising and counter-intuitive that the electrical characteristics would not be degraded since excess oxygen, particularly at the compact-wire interface and at zones where tantalum or niobium are anodized (formed) to establish a dielectric pentoxide layer, is normally associated with electrical instability.
While the reason for enhanced bonding is not fully understood it is believed that the thin oxidized layer at the surface lowers local melting point and increases vapor pressure in a limited critical zone to enable chemically enhanced sintering. At the same time the limited amount of added oxygen at the peak does not act as a source of excessive oxygen that will migrate to the powder or to the wire bulk with deleterious effects. For example, tantalum capacitor powders can have 1000-20,000 PPM, oxygen (normally at or close to 3,100 in high-grade units) and lead wires can be made with 50-300 PPM, preferably under 200 PPM. A small increase of oxygen at the wire surface as described herein has little or no potential to degrade the powder or wire electrical or physical properties under thermal and electrical stress conditions of sintering, capacitor manufacture or capacitor use. The above described oxygen depth profile of the wire changes, in the course of bonding to the compact as part of sintering the compact with embedded wire or by later welding. But there is little or no further change in the course of pyrolytic decomposition of an electrolyte precursor to form solid electrolyte (typically manganese dioxide) within the compact or in the course of capacitor usage.
The same considerations also apply to appropriate surface additives other than oxygen, e.g. niobium, germanium, silicon, carbon and others in appropriate refractory metal bonding situations. Without limiting the invention by theory of operation, it also appears that the properly selected added species of the refractory metal article surface diffuses to the additional refractory material (e.g. powder of anode) at the surface and that there is a crossing reverse diffusion of atoms of the additional refractory material that accounts for at least a substantial portion of the enhanced sintering (reduction of necessary time and/or temperature) and enhanced bond strength shown above with second order end product benefits (e.g. lower capacitor leakage) shown herein.
Illustrative examples below are devoted to tantalum and niobium wire in electrolytic capacitors, but it should be understood that the invention is applicable to other refractory metals (Ti, Mo, W, V, Zr, Hf) and uses not only in capacitors, but also in batteries, reactor systems, and other devices. The invention can also be applied to bonding of solid articles as well as bonding a solid article to a loose or consolidated powder mass.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which: